The present invention relates to a system for controlling the clutch torque of an electromagnetic clutch disposed between an engine and the transmission of a motor vehicle.
A clutch torque control system for starting of a vehicle is, for example, disclosed in Japanese Patent Laid Open No. 57-15023. In the system, the clutch current flowing in a coil of the clutch exponentially increases with respect to an increase of the engine speed for starting the vehicle. When vehicle speed reaches a predetermined speed (for example 20 km/h), a rated current (lock-up current) flows through the coil, so that the clutch is fully engaged. During deceleration of the vehicle, when the vehicle speed decreases below the predetermined speed, the clutch current is cut off to disengage the clutch.
Referring to FIG. 5 showing a clutch torque characteristic of an electromagnetic clutch, when the clutch torque exceeds a stall point P on a clutch torque line l, the clutch is substantially engaged. On the other hand, when the speed of the vehicle reaches a predetermined value (20 km/h), the clutch is engaged. If the rated current flows at a point Q above the stall point P, the clutch is smoothly engaged without a shock. However, in a transmission system combining an electromagnetic clutch with a belt-drive infinitely variable transmission, the transmission ratio is changed to a lower value, when the magnitude of depression of an accelerator pedal is small, which means a low engine speed and a low clutch torque. In such a system, when the speed of the vehicle exceeds the predetermined value at a low transmission ratio, the clutch is engaged from a low clutch torque point Q' below the stall point P. Accordingly, the clutch torque rises from the point Q' as shown by the dashed line. Since the clutch torque rapidly rises from a low clutch torque state and hence the clutch is rapidly engaged, a large shock occurs in the system, which causes undesirable problems for the vehicle.